This application claims the right of priority under 35 U.S.C. xc2xa7119 based on Japanese Patent Application No. 2002-100581, filed on Apr. 2, 2002, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.
The present invention relates to exposure apparatuses used to expose a pattern on a reticle or mask (these terms are used interchangeably in this application) onto a plate, such as a wafer and a glass plate in a manufacture process of semiconductor devices, liquid crystal display devices etc. The present invention is suitable, for example, for a scan-type exposure apparatus.
The fabrication of a device, such as a semiconductor device, an LCD device, and a thin film magnetic head using the lithography process has conventionally employed a projection exposure apparatus to transfer a circuit pattern formed on a photo-mask or reticle onto a photosensitive wafer substrate and glass plate (xe2x80x9cwaferxe2x80x9d hereinafter). Recent semiconductor devices etc. have been required for finer patterns. This demand requires higher resolution of the projection optical system. The higher resolution is available with a shorter wavelength of exposure light and a larger numerical aperture of the projection optical system.
A chip pattern size of one semiconductor device tends to become large, and an apparatus for exposing a larger area has been demanded.
These two requirements need a projection optical system with a large exposure area and high resolution. However, it becomes difficult to maintain imaging performance, such as distortion, within permissible accuracy for a larger exposure area and higher resolution throughout the exposure area.
A scan-type exposure apparatus has currently called attentions, which transfers a reticle pattern onto a wafer by scanning a slit-shaped, e.g., arc-shaped illumination area on the reticle and wafer synchronously.
This system illuminates a slit area on the reticle, and uses only part of the projection optical system. It thus has an advantage in easily maintaining the imaging performance, such as distortion, within predetermined accuracy.
An additional advantage is that the slit-shaped illumination on the reticle makes available the maximum diameter of the effective exposure area of the projection optical system, and the scan may enlarge an exposure area in the scan direction without affected by any restriction of the optical system.
However, a currently demanded further finer pattern requires the scan-type exposure apparatus to reduce distortion of a pattern image.
A reduction of distortion naturally leads to reductions of various aberrations in the projection optical system. Therefore, a projection optical system to be loaded on a conventional stepper is optically designed on condition that various aberrations and distortions are reduced averagely in the entire projection field. In order to maintain various aberrations and distortion within permissible ranges, the projection optical system is assembled by a method that repeats complex and arduous assemblies, adjustments and inspections, and the method includes the steps of processing a lens element and optical element with high precision, actually measuring various aberrations, and adjusting an air separation between two lenses, a lens""s tilt and a parallel decentering.
In particular, the above adjustment method may adjust symmetrical components or asymmetrical but regular components of the distortion among various aberrations.
However, the above adjustment method cannot adjust so-called random components disadvantageously.
Accordingly, in order to mitigate difficulties of precise manufacture of the projection optical system and maintain the random component within a designed permissible range, Japanese Laid-Open Patent Application No. 8-203805, for example, discloses a method the steps of observing a distortion characteristic of an assembled projection optical system, and inserting a polished optical correction plate or correction optical element into a projection optical path so as to partially deflect a principal ray that passes each point in the projection field so that the observed distortion characteristic at each point in the projection field may be minimized. Japanese Laid-Open Patent Application No. 8-203805 is directed to a correction method for use with a stepper using an optical correction plate.
Japanese Laid-Open Patent Application No. 11-045842 discloses a correction method using an optical correction plate in a scan-type exposure apparatus. Japanese Laid-Open Patent Application No. 11-045842 addresses that the static distortion characteristic is averaged over a width in the projection area in a scan direction and turns to a dynamic distortion characteristic, when a mask pattern is exposed onto a photosensitive substrate using a scan type projection exposure apparatus, and corrects a random component in the dynamic distortion characteristic by arranging in a projection optical path a distortion correction plate that is made through a local polishing process of a surface of a transparent parallel plate or optical correction plate.
Japanese Laid-Open Patent Application No. 11-031652 is directed to manufactures and measurements of an optical correction plate. Japanese Laid-Open Patent Application No. 11-031652 provides an optical correction plate with a plate having a wedge angle, so as to prevent interference of backlight of the optical correction plate in measuring a surface shape of an optical correction plate using an interferometer.
While the above discusses the prior art about distortion, a fluctuation of the imaging performance of an image projected by the projection optical system should be also considered in addition to distortion to create a finer pattern. The finer pattern narrows a permissible fluctuation range of the imaging performance. In order to correct a fluctuation of the imaging performance, such as a magnification and focal position, as a result of that the projection optical system absorbs the illuminated light, as disclosed in Japanese Laid-Open Patent Applications Nos. 60-78455 and 63-58349, conventional projection exposure apparatuses have included an imaging-performance correction mechanism for detecting a quantity of light incident upon the projection optical system and for correcting the fluctuation of the imaging performance of the projection optical system according to the detected quantity of light.
For example, the mechanism disclosed in Japanese Laid-Open Patent Application No. 60-78455 previously prepares a model indicative of a fluctuation characteristic in the imaging performance in a projection optical system, calculates an energy amount of light incident upon the projection optical system at certain time intervals using a photoelectric sensor, etc. on a wafer stage mounted with a wafer, and calculates a fluctuation of the imaging performance by applying an integral value of the light energy amount to the model. This method calculates the exposure time to calculate the integral value of the light energy incident upon the projection optical system by always monitoring, for example, a signal indicative of an open state of a shutter for opening and closing illumination light, obtains the fluctuation of the imaging performance of the projection optical system according to this model, and provides a correction based on this fluctuation. This appears to solve problems associated with the fluctuation of the imaging performance due to the projection optical system that has absorbed illumination light.
However, the illumination light passes through the mask, and thermally deforms the mask that absorbs the illumination light, disadvantageously deteriorating the imaging performance. In particular, the mask forms a pattern using a light-shielding film, such as a chrome film, and the light-shielding film absorbs larger heat than a glass plate that has high transmittance. More particular, the recent technology often uses the light-shielding film with low reflectance on the mask to prevent flare in the optical system, and this trend further increases the thermal absorption on the light-shielding film.
A circuit pattern using a light-shielding film on the mask does not necessarily distribute evenly on the entire mask, rather often distributes unevenly. In this case, the temperature on the mask locally rises and possibly generates an anisotropic deformation. A similar anisotropic deformation may possibly occur in exposure that uses a variable field stop (or reticle blind) etc. to expose part of a pattern on a mask. The mask""s deformation thus generated causes an anisotropic deformation in the projected image. In this case, a correction of only the magnification component is insufficient.
An even correction of the thermal deformation of the mask is difficult because a thermal deformation amount and a fluctuation of the imaging performance changes according to kinds of the mask used. For example, a fluctuation of the imaging performance due to the thermal deformation of a mask used to adjust the imaging performance at the time of shipping of the projection exposure apparatus may be corrected as if it is regarded as the fluctuating imaging performance of the projection exposure apparatus. However, an accurate correction is unavailable for other masks because these masks have different thermal deformation amounts. In particular, exposures by exchanging masks one by one would accumulate fluctuations of the imaging performance and cause a large error if a thermal deformation amount of each mask is not considered.
Japanese Laid-Open Patent Application No. 4-192317 discloses a projection exposure apparatus that corrects changing optical performance that results from the thermal deformation of a mask, while considering as parameters the thermal absorptance of chrome that forms a mask pattern, and an existence ratio of chrome in a pattern, etc. Japanese Laid-Open Patent Application No. 4-192317 proposes the correction method of the imaging performance for a full field method.
A scan exposure method scans an illuminated area on a mask, and increases considerations for the mask, such as a cooling effect of the mask associated with the mask scan, whereby the calculation of the thermal deformation amount of the mask becomes more complex than the full field method.
Regarding the scan exposure, Japanese Laid-Open Patent Application No. 10-214780 discloses a projection exposure apparatus that corrects changing optical performance caused by a thermally deforming mask. Japanese Laid-Open Patent Application No. 10-214780 detects a deformation of the mask using an oblique light projection optical system, and operates and corrects the deformation amount of the mask.
A brief description will now be given of a mechanism used for the scan exposure method disclosed in Japanese Laid-Open Patent Application No. 10-214780. An exposure beam in the scan exposure has a narrow width in the scan direction and a wide width in a direction orthogonal to the scan direction. This method addresses an irradiation range of the exposure beam, controls focusing or leveling for the deformation in the scan direction, corrects the deformation in the direction orthogonal to the scan direction, and projects a pattern practically satisfactorily. In correcting the changing optical characteristic, the piezoelectric element is driven at both ends of the mask, and the mask is deformed so as to cancel its undesired deformation. There are plural piezoelectric elements in a direction in which the mask is moved in scanning.
The deformation of the mask is detected at one point or at plural points, and the optimal deformation correction amount is operated using the averaging method or least squares method. A method for detecting deformation at plural points may provide a plurality of mask deformation detection systems or use a diffraction grating to obtain plural beams from one light source. A method has also been proposed for arranging detection points for detecting the mask""s deformation at both sides of the irradiation area of the exposure light in the scan direction of the mask, for detecting the mask""s deformation amount in front of the irradiation area of the exposure light in the scan direction of the mask, and for real-time correcting a projected image of the mask pattern according to deformation amounts of the mask in the illumination area.
Japanese Laid-Open Patent Application No. 11-045842 discloses use of a correction optical element as prior art that is directed to a correction of distortion, in particular, in a scan-type exposure apparatus. Japanese Laid-Open Patent Application No. 10-214780 discloses a correction method using deformation detection means to detect mask""s distortion and deformation.
A method disclosed in Japanese Laid-Open Patent Application No. 11-045842 uses a correction optical element and may correct distortion that results from a deformation in the projection optical system or a deformation generated at the time of holding the mask, but cannot disadvantageously correct the changing optical performance generated due to the mask""s thermal deformation etc. during exposure. It has an additional disadvantage in that when the mask is replaced, it cannot correct distortion resulting from the mask""s surface shape.
The method disclosed in Japanese Laid-Open Patent Application No. 10-214780 may measure the mask""s surface shape and handle factors that attribute to the mask, but cannot disadvantageously correct distortion caused by the projection optical system. Therefore, it is necessary to simultaneously employ both methods of Japanese Laid-Open Patent Applications Nos. 11-045842 and 10-214780.
As disclosed in Japanese Laid-Open Patent Application No. 10-214780, when the deformation detection means of an oblique light projection system is arranged at the rear surface of the mask or mask pattern surface side, and the correction optical element for correcting distortion is arranged between the mask and projection optical system, the mask, the deformation detection means, the correction optical element, and the projection optical system should be arranged in this order.
Preferably, the correction optical element is arranged closer to the mask. In order to arrange the correction optical element close to the mask and properly arrange the deformation detection means, the detection light of the deformation detection means should detect the mask pattern surface through the correction optical element.
The correction optical element is a plate optic with a certain thickness, and experiences a minute polishing process for correction of part of the surface shape. Therefore, the detection light irradiated from the illumination optical system in the detection means enters the correction optical element in an oblique direction, reflects on the mask pattern surface, re-enters the correction optical element in an oblique direction, and reaches a detection part through a light-receiving element in the detection means. The aberration occurs at this time. A description will be given of the generation principle with reference to FIG. 3. FIG. 3A is a section of the reticle surface position detection system in a measurement direction. A pattern image for measurement that has been illuminated by an illumination optical system (not shown) is projected on a pattern surface on a reticle R through a light-projecting optical system and a correction optical element G1, and a reflected pattern image is imaged again on a light-receiving element 44 through a correction optical element G1 and light-receiving optical system. As shown in FIG. 3A, a correction optical element G1 as a parallel plate is inserted while inclined in condensed light in an observation optical system of an oblique light projection system.
FIG. 3B shows a section in a non-measurement direction. As shown in FIG. 3B, when the section in the non-measurement direction is addressed, the correction optical element G1 is arranged in the optical path of the light-projecting and light-receiving optical systems, perpendicular to the optical axis.
This enlarges a refractive angle of light having an angle of aperture between the measurement and non-measurement sections, and generates astigmatism that causes a positional offset between imaging points in the measurement and non-measurement directions. In particular, a detection optical system that combines the light-projecting and light-receiving optical systems with each other doubles the astigmatism on the light-receiving and re-imaging surface.
Since this method detects surface positions and conditions by projecting a slit-shaped mark as a measurement mark onto a pattern surface on the reticle R and detecting a positional offset of the mark using the light-receiving element 44, the astigmatism generated at the image-surface side decreases the resolution of the observing light-receiving element and disadvantageously deteriorates a measurement precision.
The correction optical element in the prior art is dimensioned so as to maintain an area through which the illumination light that has exposed the mask transmits. Therefore, the detection means of an oblique projection system detects a mask pattern surface through the correction optical element, and an area to detect the mask pattern surface through the correction optical element disadvantageously decreases by the oblique angle.
While FIG. 3 discusses the astigmatism, there is another problem in that a chromatic aberration occurs and lowers the detection precision as a result of an insertion of the correction optical element as a parallel plate in the convergence light.
Dispersion and thus chromatic aberration occur due to different refractive indexes among wavelengths of light used as detection light.
Accordingly, it is an exemplified object of the present invention to provide an exposure apparatus for projecting a mask pattern properly by detecting a mask""s deformation precisely while preventing the apparatus from being larger.
An exposure apparatus of one aspect of the present invention includes a projection optical system for projecting a pattern formed a mask onto an object to be exposed, a correction optical element, provided between the mask and the projection optical system, for reducing a deformation of the pattern, and a detector of an oblique light projection system, provided at a side of a pattern surface of the mask, for detecting a surface shape of the mask through the correction optical element.
The correction optical element may be a correction plate for correcting an offset of the surface shape of the mask from an ideal plane. Alternatively, the correction optical element may be a correction plate for correction optical performance of the projection optical system. The detector may include a correction optical system for correcting optical aberration associated with detections through the correction optical element. The optical aberration may be astigmatism and/or chromatic aberration.
The detector may include a light-irradiating part that includes an illumination optical system for irradiating light onto a mask in an oblique direction through the correction optical element, and the correction optical system, a light-receiving part that includes an imaging optical system that receives, through the correction optical element, light that has been irradiated by the light-irradiating part and reflected on the mask, and the correction optical system, and a photodetector for outputting a detection signal corresponding to a position of the reflected light received by the light-receiving part.
The photodetector may detect the surface shape of the mask at plural measurement points. The exposure apparatus may be a scan-type exposure apparatus, and the plural measurement points may be arranged in a direction orthogonal to a scan direction of the mask.
The correction optical element has such a size as transmits illumination light for illuminating the mask during exposure, and enables entire detection light by said detector to pass through the correction optical element.
The exposure apparatus may be a scan-type exposure apparatus.
The exposure apparatus may be a scan-type exposure apparatus, and scan the correction optical in synchronization with the mask.
A device fabrication method according another aspect of the present invention includes the steps of exposing a pattern on a reticle onto an object by using the above exposure apparatus, and performing a predetermined process for the exposed object. Claims for the device fabrication method that exhibits operations similar to those of the above exposure apparatus cover devices as their intermediate products and finished products. Moreover, such devices include semiconductor chips such as LSIs and VLSIs, CCDs, LCDs, magnetic sensors, thin-film magnetic heads, etc.
Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings.